A group of 48 weanling Osborne-Mendel rats were fed diet containing 56% sucrose. Each rat was caged singly with a donor rat that was infected with Streptococcus mutans 6715-15. The recipients and donors were swabbed twice a week and the bacteria cultured on Mitis Salivarius agar. Every two weeks subgroups of 12 donors and 12 recipients were sacrificed. The teeth were evaluated for caries and serum and saliva collected for antibody assay. Salivary IgA and IgG and serum IgG and IgM antibodies reactive with whole cells (WC), glucosyltransferase (GTF) and the serotype antigen (d) were present before the animals became infected with S. mutans. Serum antibodies reactive with these antigens increased as a result of infection whereas antibodies in saliva were either unchanged or reduced. The apparent reduction in salivary antibody may result from antibody binding to the bacteria and being unavailable for assay. Cross-absorption studies suggest that the presence of S. Salivarius may account for antibody reactive with S. mutans in the baseline samples. Those results suggest that infection is capable of inducing an immune response but that cross-reactive antibody may be important in controlling initial colonization of S. mutans.